1. Field of the Invention
The present invention relates to a semiconductive rubber composition and a semiconductive rubber roller made of the semiconductive rubber composition to be assembled into an electrophotographic apparatus such as a laser printer.
2. Description of Related Art
An electrophotographic apparatus such as a laser printer, an electrostatic copier, a plain paper facsimile or a composite apparatus thereof has been widely spread with improvements in speed and picture quality, colorization and downsizing, for example. Such improvements are still in progress at the present time.
For example, the laser printer is in the process of research and development for downsizing aiming at further universalization and implementation of maintenance-free properties through improvements in durability of respective portions. With this stream, further downsizing and high durability are required also to rubber components such as a semiconductive rubber roller assembled into an image forming apparatus.
The semiconductive rubber roller is employed in the laser printer as a charging roller, a developing roller, a transfer roller or a cleaning roller, for example. The developing roller is employed for transporting a toner brought into contact with the surface of the developing roller at high pressure by a quantity regulating blade and charged in a developing section of the laser printer or the like to a surface of a photosensitive drum and developing an electrostatic latent image formed on the surface into a toner image.
In a general laser printer, the developing roller is detachably mounted on a casing of the laser printer as a cartridge integrated with the photosensitive drum and a toner container. If the toner stored in the toner container is used up, the developing roller and the photosensitive drum are also exchanged for new ones as the cartridge, so that the laser printer is maintenance-free.
In order to satisfy the recent requirement for further downsizing of the laser printer or to develop a miniature full-color laser printer, the cartridge must be more downsized than ever.
Therefore, the developing roller must be:                more reduced in diameter than ever;        reduced in hardness and increased in flexibility to be brought into contact with the surface of the photosensitive drum in a state maintaining a nip thickness equivalent to that of a conventional developing roller upon reduction in diameter; and        prevented from the so-called “permanent set” through which the developing roller is hardly restored to the original state when released from the pressure contact with the photosensitive drum after deformation following the pressure contact by maintaining a state having small compression set upon reduction in hardness thereby preventing unevenness of imaging resulting from the permanent set.        
More specifically, the developing roller must have hardness of not more than A60 in the type A durometer hardness defined in JIS (Japanese Industrial Standards) K6253:2006 “Rubber, vulcanized or thermoplastic—determination of hardness” and compression set of not more than 5% defined in JIS K6262:2006 “Rubber, vulcanized or thermoplastic determination of compression set at ambient, elevated or low temperatures”. In this specification, the type A durometer hardness is expressed in a value measured in an environment having a temperature of 23±1° C. and relative humidity of 55±1%. Further, the compression set is expressed in a value measured at a temperature of 70±1° C. with compressibility of 25% for a measuring time of 22 hours.
When a cartridge having been warehoused is mounted on the laser printer for starting image formation or image formation is started or restarted from a state stopping the laser printer while the developing roller is in pressure contact with the surface of the photosensitive drum, for example, the permanent set is linearly caused on a portion of the outer peripheral surface of the developing roller having been in pressure contact with the surface of the photosensitive drum during the warehousing or the stoppage along a generatrix of a cylinder forming the outer peripheral surface of the roller.
If the developing roller is not immediately released from the permanent set upon starting or restarting of the image formation, a position of a formed image corresponding to the portion of the developing roller causing the permanent set is reduced in density, to result in striped unevenness of the density, i.e., unevenness of imaging in the formed image. Not only when the compression set is excessive but also when the hardness is excessive, the developing roller may not be immediately released from permanent set particularly in an initial stage upon starting or restarting of the image formation, to cause unevenness of imaging.
The semiconductive rubber roller such as the developing roller is made of a semiconductive rubber composition containing copolymer rubber containing ethylene oxide as a copolymeric component and exhibiting ion conductivity, for example. The copolymer rubber exhibiting ion conductivity can be prepared from epichlorohydrin-based copolymer rubber such as epichlorohydrin-ethylene oxide copolymer rubber or epichlorohydrin-ethylene oxide-allyl glycidyl ether copolymer rubber or polyether-based copolymer rubber such as ethylene oxide-allyl glycidyl ether copolymer rubber, ethylene oxide-propylene oxide copolymer rubber or ethylene oxide-propylene oxide-allyl glycidyl ether copolymer rubber, for example.
A semiconductive rubber roller is obtained by preparing a semiconductive rubber composition by blending chloroprene rubber (CR), acrylonitrile butadiene copolymer rubber (NBR) or the like into the copolymer rubber if necessary while adding additives such as a vulcanizer and a vulcanization accelerator, molding the semiconductive rubber composition into the form of a roller and thereafter vulcanizing the roller (refer to Patent Document 1: Japanese Unexamined Patent Publication No. 2008-116684, Patent Document 2: Japanese Unexamined Patent Publication No. 2007-313883 and Patent Document 3: Japanese Unexamined Patent Publication No. 2007-286236).
According to studies made by the inventor, however, it has been proven that each of semiconductive rubber rollers described in Patent Documents 1 to 3 easily causes permanent set due to large compression set when the hardness thereof is reduced in order to maintain a prescribed nip thickness upon reduction in diameter, and easily causes unevenness of imaging due to the permanent set when the semiconductive rubber roller is used as a developing roller for repeating image formation, for example.